Various methods for separating target gas (product gas) from material gas are known, and pressure swing adsorption (PSA) is one of such methods. Since a PSA process can be performed easily at a relatively low cost, it is widely utilized in the related field. The PSA process typically utilizes a plurality of adsorption towers loaded with an adsorbent. After material gas is introduced in each of the adsorption towers, the process steps of adsorption, decompression, desorption and pressurization are repeated to obtain a targeted product gas.
Specifically, the target gas is obtained on a principle described below. When the pressure of material gas introduced in the adsorption tower is increased, the partial pressure of an unnecessary gas component contained in the material gas also increases. As a result, the unnecessary gas component is adsorbed by the adsorbent loaded in the adsorption tower. (That is, the unnecessary gas component is removed from the material gas.) In this state, the gas in the adsorption tower is discharged as target gas (product gas) containing little amount of unnecessary gas. Thereafter, as the pressure in the adsorption tower drops, the unnecessary gas component is desorbed from the adsorbent (regeneration of the adsorbent) The desorbed component together with other components remaining in the tower are then discharged from the tower. The regenerated adsorbent can be utilized for removing an unnecessary gas component from newly introduced material gas for obtaining an additional amount of target gas. Examples of target gases include hydrogen gas, oxygen gas and nitrogen gas and the like.
The kind of adsorbent to be used in each of the adsorbent towers is selected based on the kind of a target gas and the kind of an unnecessary gas component to be removed. For example, zeolite is conventionally used as the adsorbent for removing nitrogen component and carbon monoxide component from material gas for obtaining hydrogen gas as a product gas. On the other hand, an activated carbon-based adsorbent is used for removing carbon dioxide component from material gas.
For the PSA process, various improvements have been made for enhancing the purity of the obtained target gas and the yield. These improvements are disclosed in the gazettes of JP-B2-62(1987)-38014, JP-B2-7(1995)-4498 and JP-A-8(1996)-10551.
As one of the improvements for the PSA process, a technique is developed for increasing the regeneration efficiency of an adsorbent. Specifically, it is now assumed that a desorption step is finished in one adsorption tower (first adsorption tower) while an adsorption step is being performed in another adsorption tower (second adsorption tower). At that time, product gas is introduced from the second adsorption tower to the first adsorption tower. As a result, the gas remaining in the first adsorption tower is discharged, i.e. the first adsorption tower can be cleaned (cleaning step). Such cleaning can increase the regeneration efficiency of the adsorbent loaded in the adsorption tower, which may result in an increase in the yield of the hydrogen gas.
Another improvement for the PSA process is as follows. It is now assumed that an adsorption step is finished in a first adsorption tower and the internal pressure of the adsorption tower is high, while a desorption step (or cleaning step) is finished in a second adsorption tower and the internal pressure of the adsorption tower is low. In this state, the remaining gas is introduced from the first adsorption tower (at high pressure) to the second adsorption tower (at low pressure) for equalizing the internal pressures of the two adsorption towers. This technique is advantageous in that decompression of the first adsorption tower and pressurization of the second adsorption tower can be performed easily at the same time.
The PSA process improved as described above can be performed using a separation apparatus X as shown in FIG. 1. The separation apparatus X includes three adsorption towers A-C, a material gas pipe 1, a product gas pipe 2, a remaining gas outlet pipe 3, a remaining gas inlet pipe 4, a product purge pipe 5 and a discharge pipe 6. The pipes 1-6 are provided with automatic valves a-p. The remaining gas outlet pipe 3 and the product purge pipe 5 are provided with flow rate controlling valves 7 and 8, respectively. The above-described five process steps (adsorption, decompression, desorption, pressurization and cleaning) are performed in each of the adsorption towers A-C by selectively opening or closing the automatic valves a-p.
As shown in FIG. 9, the five process steps are performed in the respective adsorption towers A-C at different timings. In the example shown in FIG. 9, nine process steps are defined. For example, in a first step (S1), an adsorption step (second adsorption step) is performed in the adsorption tower A, a pressurization step (first pressurization step) is performed in the adsorption tower B, and a desorption step is performed in the adsorption tower C. At that time, each of the automatic valves (Va-Vp) is open (o) or closed (x).
The gas flow in the separation apparatus X varies in each process step. FIGS. 10A-10I illustrate variations of the gas flow. Specifically, as shown in FIG. 10A, in the first step (S1), material gas is introduced into the adsorption tower A through the material gas pipe 1 and the automatic valve a. In the adsorption tower A, unnecessary gas components are removed by the adsorbent and product gas is discharged from the tower. The product gas is partially collected through the automatic valve i and the product gas pipe 2 while partially introduced into the adsorption tower B through the product purge pipe 5, the automatic valve p, the flow rate controlling valve 8, the remaining gas inlet pipe 4 and the automatic valve j. As a result, pressure in the adsorption tower B is raised. The amount of product gas introduced in the adsorption tower B is controlled by the flow rate controlling valve 8. From the adsorption tower C, the gas remaining in the tower is discharged through the automatic valve f and the discharge pipe 6.
In the second step (S2), an adsorption step (third adsorption step), a pressurization step (second pressurization step) and a cleaning step are performed in the adsorption towers A, B and C, respectively. Specifically, as shown in FIG. 10B, the adsorption step is performed in the adsorption tower A subsequent to the introduction of material gas. The product gas thus obtained is discharged from the adsorption tower A. The discharged product gas is partially collected while partially introduced into the adsorption towers B and C. The pressure in the adsorption tower B is raised by the introduction of the product gas. The product gas is introduced into the adsorption tower C through the product purge pipe 5, the automatic valve p, the flow rate controlling valve 8, the remaining gas inlet pipe 4 and the automatic valve m. As a result, remaining gas is discharged from the adsorption tower C. At that time, it is preferable that the product gas introduced into the adsorption tower C is not discharged and only the remaining gas is discharged. This is based on the recognition that the collection of product gas is difficult once the product gas is discharged from the tower. In a prior art method, therefore, the amount of product gas introduced into the adsorption tower C is set to be smaller than the volume of the adsorbent loaded in the adsorption tower C (as converted into volume at common temperature and under atmospheric pressure).
In the third step (S3), a decompression step (first pressure equalization step), an adsorption step (first adsorption step) and a pressurization step (second pressure equalization step) are performed in the adsorption tower A, B, and C, respectively. Specifically, as shown in FIG. 10C, remaining gas discharged from the adsorption tower A is introduced into the adsorption tower C through the automatic valve h, the remaining gas outlet pipe 3, the flow rate controlling valve 7, the remaining gas inlet pipe 4 and the automatic valve m. As a result, the decompression for the adsorption tower A and the pressurization for the adsorption tower C are performed at the same time. Material gas is introduced into the adsorption tower B through the material gas pipe 1 and the automatic valve c. The adsorbent loaded in the adsorption tower B removes unnecessary gas components from the material gas for providing product gas. The product gas is discharged from the adsorption tower B and then collected through the automatic valve 1 and the product gas pipe 2.
In the fourth through the sixth steps (S4-S6), process steps described below are performed in each of the adsorption towers. In the adsorption tower A, a desorption step, a cleaning step and a pressurization step (second pressure equalization step) are performed. These process steps are similar to those performed in the adsorption tower C in the first through the third steps. In the adsorption tower B, an adsorption step (second adsorption step), an adsorption step (third adsorption step) and a decompression step (first pressure equalization step) are performed. These process steps are similar to those performed in the adsorption tower A in the first through the third steps. In the adsorption tower C, a pressurization step (first pressurization step), a pressurization step (second pressurization step) and an adsorption step (first adsorption step) are performed. These process steps are similar to those performed in the adsorption tower B in the first through the third steps.
In the seventh through the ninth steps, the process steps described below are performed in each of the adsorption towers. In the adsorption tower A, a pressurization step (first pressurization step), a pressurization step (second pressurization step) and an adsorption step (first adsorption step) are performed. These process steps are similar to those performed in the adsorption tower B in the first through the third steps. In the adsorption tower B, a desorption step, a cleaning step and a pressurization step (second pressure equalization step) are performed. These process steps are similar to those performed in the adsorption tower C in the first through the third steps. In the adsorption tower C, an adsorption step (second adsorption step), an adsorption step (third adsorption step) and a decompression step (first pressure equalization step) are performed. These process steps are similar to those performed in the adsorption tower A in the first through the third steps.
By repetitively performing the above-described first through ninth steps in each of the adsorption towers A-C, unnecessary gas components are removed from the material gas, thereby providing product gas containing a high concentration of hydrogen.
As described above, in the prior art PSA process, product gas is introduced from an adsorption tower (e.g. the adsorption tower A in the second step) in which adsorption is being performed to another adsorption tower (the adsorption tower C in the second step) in which desorption is finished for cleaning this adsorption tower. To avoid wasteful discharging of product gas, the amount of product gas introduced is set to be smaller than the volume of the loaded adsorbent. Further, in the prior art process, for efficiently utilizing high-pressure gas in an adsorption tower, internal pressure equalization is performed between an adsorption tower in which adsorption is finished (e.g. the adsorption tower A in the third step) and an adsorption tower in which adsorption is to be performed (the adsorption tower C in the third step).
Various improvements have been proposed also for adsorbents for the PSA process. For example, JP-A-10(1998)-212103 discloses zeolite having high adsorptivity for nitrogen gas and carbon monoxide gas for removing these gas components from material gas. The zeolite has a faujasite structure with a Si/Al ratio lying in the range of 1 to 3 and with a lithium-exchange ratio of no less than 70%.
As described above, the prior art PSA process has been improved in various ways. However, in spite of such improvements, the conventional PSA process still has the following problems to be solved.
The first problem relates to the yield of target gas. Conventionally, as described above, each of adsorption towers is cleaned using product gas for the purpose of enhancing the regeneration efficiency of the adsorbent and the yield of target gas. Actually, however, it is found that the yield is not increased as much as expected.
The second problem is caused by the pressure equalization step between two adsorption towers. As described above, by introducing remaining gas from an adsorption tower on the high pressure side to an adsorption tower on the low pressure side, target gas included in remaining gas can be collected. However, the remaining gas contains not only the target gas but also unnecessary gas components. Therefore, part of the unnecessary gas components adsorbs to the adsorbent in the adsorption tower to which the remaining gas is introduced, so that the adsorbent cannot exhibit as much adsorption effect as expected.
The third problem is an increase in size of the apparatus due to the use of a plurality of adsorbents. As the material gas for the PSA process, use may be made of mixed gas obtained by steam-reforming hydrocarbon (methanol or natural gas), for example. For example, in the case of reforming methanol, the composition of the mixed gas is about 75% hydrogen gas, about 24% carbon dioxide gas and about 1% carbon monoxide gas. To obtain high purity hydrogen gas (target gas) from such mixed gas, both of carbon dioxide component and carbon monoxide component need be removed. As described above, in the prior art PSA process, zeolite is used as the adsorbent for removing carbon monoxide component, whereas activated carbon-based adsorbent is used for removing carbon dioxide component. Therefore, to remove both carbon dioxide component and carbon monoxide component, the two kinds of adsorbents need be loaded in each of the adsorption towers. To load the plural kinds of adsorbents, adsorption towers of large capacity need be used, which disadvantageously increase the size of the entire separation apparatus.
The reason why two kinds of adsorbents are needed for removing carbon dioxide and carbon monoxide is as follows.
As described above, the PSA process is a gas separation method which utilizes the fact that the amount of an unnecessary gas component adsorbed varies in accordance with the pressure variation in the adsorption tower. Therefore, to effectively remove an unnecessary gas component in the PSA process, a condition need be satisfied that the unnecessary gas component is likely to be adsorbed by the adsorbent under high pressure while it is unlikely to be adsorbed (i.e. is likely to be desorbed) under low pressure. However, when a prior art zeolite-based adsorbent is used for carbon dioxide, this condition is not satisfied. This point will be described below in detail with reference to FIG. 17.
The graph in FIG. 17 shows how adsorption isotherms (25° C.) for carbon dioxide gas become when three kinds of adsorbents (a 85% Li-exchanged zeolite, a Ca-exchanged A-type zeolite and a carbon-based adsorbent) are used. The signs “Li85Z”, “CaAZ” and “Car.” in the figure indicate the 85% Li-exchange zeolite, the Ca-exchange A-type zeolite and the carbon-based adsorbent, respectively. The 85% Li-exchange zeolite has a faujasite structure, a Si/Al ratio of 1 and a lithium-exchange ratio of 85%. In the graph of FIG. 17, the abscissa is adsorption equilibrium pressure (A.E.P.), whereas the ordinate is adsorbed amount of carbon dioxide (CO2 Ad.)
The following is understood from the graph. As the equilibrium adsorption pressure increases, the amount of carbon dioxide adsorbed by the carbon-based adsorbent increases generally linearly. On the other hand, in the case of 85% Li-exchange zeolite and Ca-exchange A-type zeolite, the adsorbed amount of carbon dioxide rapidly increases initially but becomes generally constant when a certain value is exceeded. Specifically, in the case of 85% Li-exchange zeolite, the increasing rate of the adsorbed amount becomes small from when the equilibrium adsorption pressure exceeds approximately 1000 Torr. In the case of Ca-exchange A-type zeolite, the increasing rate of the adsorbed amount becomes small from when the equilibrium adsorption pressure exceeds approximately 750 Torr.
From the above, it is understood that the 85% Li-exchange zeolite and the Ca-exchange A-type zeolite are not suitable for removing carbon dioxide component in the PSA process, although the carbon-based adsorbent is effective for the removal. This point will be described using, as an example, a mixed gas containing about 75% hydrogen gas, about 24% carbon dioxide gas and about 1% carbon monoxide gas. For example, when the adsorption pressure for the mixed gas is set to 0.8 MPa and the desorption pressure for the gas is set to ⅛ (approximately equal to atmospheric pressure) of the adsorption pressure, the partial pressure of carbon dioxide gas contained in the mixed gas becomes about 0.192 MPa (1440 Torr) during adsorption and about 0.024 MPa (180 Torr) during desorption. As will be understood from the graph of FIG. 17, in the case where the carbon-based adsorbent is used, the adsorbed amount is 66 ml/g when the partial pressure of carbon dioxide gas is 1440 Torr whereas the adsorption amount is 18 ml/g when the partial pressure is 180 Torr. This indicates that 48(=66−18) ml/g of carbon dioxide gas is removed by varying the partial pressure of carbon dioxide gas in the range of 180 to 1440 Torr.
In the case where the Ca-exchange A-type zeolite is used, the adsorbed amount is 85 ml/g when the partial pressure of carbon dioxide gas is 1440 Torr whereas the adsorbed amount is 48 ml/g when the partial pressure is 180 Torr. Therefore, the amount of carbon dioxide gas removed is 37(=85−48) ml/g. In the case where the 85% Li-exchange zeolite is used, the adsorbed amount is 119 ml/g when the partial pressure of carbon dioxide gas is 1440 Torr whereas the adsorbed amount is 82 ml/g when the partial pressure is 180 Torr. Therefore, the amount of carbon dioxide gas removed is 37(=119−82)ml/g.
In this way, a larger amount of carbon dioxide gas can be removed by the carbon-based adsorbent than by the zeolite-based adsorbent. Conventionally, therefore, a zeolite-based adsorbent has been considered to be unsuitable for the removal of carbon dioxide component in the PSA process.